Frequently Asked Questions

Is the plan to use GitHub as a package repository?

No. The plan for Cargo is to use crates.io, like npm or Rubygems do with npmjs.com and rubygems.org.

We plan to support git repositories as a source of packages forever, because they can be used for early development and temporary patches, even when people use the registry as the primary source of packages.

Why build crates.io rather than use GitHub as a registry?

We think that it’s very important to support multiple ways to download packages, including downloading from GitHub and copying packages into your package itself.

That said, we think that crates.io offers a number of important benefits, and will likely become the primary way that people download packages in Cargo.

For precedent, both Node.js’s npm and Ruby’s bundler support both a central registry model as well as a Git-based model, and most packages are downloaded through the registry in those ecosystems, with an important minority of packages making use of git-based packages.

Some of the advantages that make a central registry popular in other languages include:

• Discoverability. A central registry provides an easy place to look for existing packages. Combined with tagging, this also makes it possible for a registry to provide ecosystem-wide information, such as a list of the most popular or most-depended-on packages.
• Speed. A central registry makes it possible to easily fetch just the metadata for packages quickly and efficiently, and then to efficiently download just the published package, and not other bloat that happens to exist in the repository. This adds up to a significant improvement in the speed of dependency resolution and fetching. As dependency graphs scale up, downloading all of the git repositories bogs down fast. Also remember that not everybody has a high-speed, low-latency Internet connection.

Will Cargo work with C code (or other languages)?

Yes!

Cargo handles compiling Rust code, but we know that many Rust packages link against C code. We also know that there are decades of tooling built up around compiling languages other than Rust.

Our solution: Cargo allows a package to specify a script (written in Rust) to run before invoking rustc. Rust is leveraged to implement platform-specific configuration and refactor out common build functionality among packages.

Can Cargo be used inside of make (or ninja, or …)

Indeed. While we intend Cargo to be useful as a standalone way to compile Rust packages at the top-level, we know that some people will want to invoke Cargo from other build tools.

We have designed Cargo to work well in those contexts, paying attention to things like error codes and machine-readable output modes. We still have some work to do on those fronts, but using Cargo in the context of conventional scripts is something we designed for from the beginning and will continue to prioritize.

Does Cargo handle multi-platform packages or cross-compilation?

Rust itself provides facilities for configuring sections of code based on the platform. Cargo also supports platform-specific dependencies, and we plan to support more per-platform configuration in Cargo.toml in the future.

In the longer-term, we’re looking at ways to conveniently cross-compile packages using Cargo.

Does Cargo support environments, like production or test?

We support environments through the use of profiles to support:

• environment-specific flags (like -g --opt-level=0 for development and --opt-level=3 for production).
• environment-specific dependencies (like hamcrest for test assertions).
• environment-specific #[cfg]
• a cargo test command

Does Cargo work on Windows?

Yes!

All commits to Cargo are required to pass the local test suite on Windows. If you encounter an issue while running on Windows, we consider it a bug, so please file an issue.

Why have Cargo.lock in version control?

While cargo new defaults to tracking Cargo.lock in version control, whether you do is dependent on the needs of your package.

The purpose of a Cargo.lock lockfile is to describe the state of the world at the time of a successful build. Cargo uses the lockfile to provide deterministic builds at different times and on different systems, by ensuring that the exact same dependencies and versions are used as when the Cargo.lock file was originally generated.

Deterministic builds help with

• Running git bisect to find the root cause of a bug
• Ensuring CI only fails due to new commits and not external factors
• Reducing confusion when contributors see different behavior as compared to other contributors or CI

Having this snapshot of dependencies can also help when projects need to be verified against consistent versions of dependencies, like when

• Verifying a minimum-supported Rust version (MSRV) that is less than the latest version of a dependency supports
• Verifying human readable output which won’t have compatibility guarantees (e.g. snapshot testing error messages to ensure they are “understandable”, a metric too fuzzy to automate)

However, this determinism can give a false sense of security because Cargo.lock does not affect the consumers of your package, only Cargo.toml does that. For example:

• cargo install will select the latest dependencies unless --locked is passed in.
• New dependencies, like those added with cargo add, will be locked to the latest version

The lockfile can also be a source of merge conflicts.

For strategies to verify newer versions of dependencies via CI, see Verifying Latest Dependencies.

Can libraries use * as a version for their dependencies?

As of January 22nd, 2016, crates.io rejects all packages (not just libraries) with wildcard dependency constraints.

While libraries can, strictly speaking, they should not. A version requirement of * says “This will work with every version ever”, which is never going to be true. Libraries should always specify the range that they do work with, even if it’s something as general as “every 1.x.y version”.

Why Cargo.toml?

As one of the most frequent interactions with Cargo, the question of why the configuration file is named Cargo.toml arises from time to time. The leading capital-C was chosen to ensure that the manifest was grouped with other similar configuration files in directory listings. Sorting files often puts capital letters before lowercase letters, ensuring files like Makefile and Cargo.toml are placed together. The trailing .toml was chosen to emphasize the fact that the file is in the TOML configuration format.

Cargo does not allow other names such as cargo.toml or Cargofile to emphasize the ease of how a Cargo repository can be identified. An option of many possible names has historically led to confusion where one case was handled but others were accidentally forgotten.

How can Cargo work offline?

Cargo is often used in situations with limited or no network access such as airplanes, CI environments, or embedded in large production deployments. Users are often surprised when Cargo attempts to fetch resources from the network, and hence the request for Cargo to work offline comes up frequently.

Cargo, at its heart, will not attempt to access the network unless told to do so. That is, if no crates come from crates.io, a git repository, or some other network location, Cargo will never attempt to make a network connection. As a result, if Cargo attempts to touch the network, then it’s because it needs to fetch a required resource.

Cargo is also quite aggressive about caching information to minimize the amount of network activity. It will guarantee, for example, that if cargo build (or an equivalent) is run to completion then the next cargo build is guaranteed to not touch the network so long as Cargo.toml has not been modified in the meantime. This avoidance of the network boils down to a Cargo.lock existing and a populated cache of the crates reflected in the lock file. If either of these components are missing, then they’re required for the build to succeed and must be fetched remotely.

As of Rust 1.11.0, Cargo understands a new flag, --frozen, which is an assertion that it shouldn’t touch the network. When passed, Cargo will immediately return an error if it would otherwise attempt a network request. The error should include contextual information about why the network request is being made in the first place to help debug as well. Note that this flag does not change the behavior of Cargo, it simply asserts that Cargo shouldn’t touch the network as a previous command has been run to ensure that network activity shouldn’t be necessary.

The --offline flag was added in Rust 1.36.0. This flag tells Cargo to not access the network, and try to proceed with available cached data if possible. You can use cargo fetch in one project to download dependencies before going offline, and then use those same dependencies in another project with the --offline flag (or configuration value).

For more information about vendoring, see documentation on source replacement.

Why is Cargo rebuilding my code?

Cargo is responsible for incrementally compiling crates in your project. This means that if you type cargo build twice the second one shouldn’t rebuild your crates.io dependencies, for example. Nevertheless bugs arise and Cargo can sometimes rebuild code when you’re not expecting it!

We’ve long wanted to provide better diagnostics about this but unfortunately haven’t been able to make progress on that issue in quite some time. In the meantime, however, you can debug a rebuild at least a little by setting the CARGO_LOG environment variable:

$ CARGO_LOG=cargo::core::compiler::fingerprint=info cargo build

This will cause Cargo to print out a lot of information about diagnostics and rebuilding. This can often contain clues as to why your project is getting rebuilt, although you’ll often need to connect some dots yourself since this output isn’t super easy to read just yet. Note that the CARGO_LOG needs to be set for the command that rebuilds when you think it should not. Unfortunately Cargo has no way right now of after-the-fact debugging “why was that rebuilt?”

Some issues we’ve seen historically which can cause crates to get rebuilt are:

• A build script prints cargo::rerun-if-changed=foo where foo is a file that doesn’t exist and nothing generates it. In this case Cargo will keep running the build script thinking it will generate the file but nothing ever does. The fix is to avoid printing rerun-if-changed in this scenario.

• Two successive Cargo builds may differ in the set of features enabled for some dependencies. For example if the first build command builds the whole workspace and the second command builds only one crate, this may cause a dependency on crates.io to have a different set of features enabled, causing it and everything that depends on it to get rebuilt. There’s unfortunately not really a great fix for this, although if possible it’s best to have the set of features enabled on a crate constant regardless of what you’re building in your workspace.

• Some filesystems exhibit unusual behavior around timestamps. Cargo primarily uses timestamps on files to govern whether rebuilding needs to happen, but if you’re using a nonstandard filesystem it may be affecting the timestamps somehow (e.g. truncating them, causing them to drift, etc). In this scenario, feel free to open an issue and we can see if we can accommodate the filesystem somehow.

• A concurrent build process is either deleting artifacts or modifying files. Sometimes you might have a background process that either tries to build or check your project. These background processes might surprisingly delete some build artifacts or touch files (or maybe just by accident), which can cause rebuilds to look spurious! The best fix here would be to wrangle the background process to avoid clashing with your work.

If after trying to debug your issue, however, you’re still running into problems then feel free to open an issue!

What does “version conflict” mean and how to resolve it?

failed to select a version for x which could resolve this conflict

Have you seen the error message above?

This is one of the most annoying error message for Cargo users. There are several situations may lead us to a version conflict. Below we’ll walk through possible causes and provide diagnostic techniques to help you out there:

• The project and its dependencies use links to repeatedly link the local library. Cargo forbids linking two packages with the same native library, so even with multiple layers of dependencies it is not allowed. In this case, the error message will prompt: Only one package in the dependency graph may specify the same links value, you may need to manually check and delete duplicate link values. The community also have conventions in place to alleviate this.

• When depending on different crates in the project, if these crates use the same dependent library, but the version used is restricted, making it impossible to determine the correct version, it will also cause conflicts. The error message will prompt: all possible versions conflict with previously selected packages. You may need to modify the version requirements to make them consistent.

• If there are multiple versions of dependencies in the project, when using direct-minimal-versions, the minimum version requirements cannot be met, which will cause conflicts. You may need to modify version requirements of your direct dependencies to meet the minimum SemVer version accordingly.

• If the dependent crate does not have the features you choose, it will also cause conflicts. At this time, you need to check the dependent version and its features.

• Conflicts may occur when merging branches or PRs, if there are non-trivial conflicts, you can reset all “yours” changes, fix all other conflicts in the branch, and then run some cargo command (like cargo tree or cargo check), which should re-update the lockfile with your own local changes. If you previously ran some cargo update commands in your branch, you can re-run them that this time. The community has been looking to resolve merge conflicts with Cargo.lock and Cargo.toml using a custom merge tool.